5G took some fifteen years to develop, starting around 2004 at university research labs. When 3GPP Release 15, the first 5G new radio (5GNR) standard, was approved in late 2017, the race to deploy began. That includes mmWave Frequencies starting around 24 GHz. At roughly the same time, research labs began looking at frequencies above 100 GHz in what might become 6G. Indeed, the second 6G Summit is coming in March 2020.

Several universities around the world have begun looking at frequencies from 100 GHz to 300 GHz as the basis for 6G. While 6G is 12 to 15 years away, we know that despite the bandwidth, latency, and reliability advances of 5G, we will eventually need still better and faster communications.

Researchers at the University of California Berkeley, Santa Barbara, and San Diego are heavily involved in 6G research. The UC Santa Barbara’s ComSenTer (Communications Sensing TeraHertz), has researchers in California and at other universities looking at ICs, circuits, and systems for 6G. 5TGW spoke with ComSenTer Director Prof. Mark Rodwell of UCSB and Co-Director Prof. Ali Niknejad of UC Berkeley to discuss the current state of 6G research.

5GTW: When did you realize that it was possible to develop circuits that operated at frequencies higher than those allocated for 5G?
Niknejad: Let me start with 5G. We started some 19 years ago with funding from the DARPA TEAM program. ago with funding from the DARPA TEAM program. At the Berkeley Wireless Research Center (BWRC) we envisioned that silicon based SiGe and CMOS technology could become a viable technology for mmWaves. For example, we saw the trend lines for CMOS crossing 100 GHz that could enable building circuits at 60 GHz. Since no one had made 60 GHz CMOS circuits, we started a measurement-based modeling methodology for transistors and passives. At ISSCC 2004, we demonstrated 60 GHz building blocks using 130 nm technology. We kept working on CMOS 60 GHz devices such as phased arrays and power amplifiers. We were the first university to jump on the idea of silicon mmWave technnology, a few years before the standards committees started talking about it. But we had in mind future generations of WiFi, or fixed point-to-point links.  We in the research community thought it sounded crazy to use mmWave for mobile communication. We had an arrangement with Samsung to run the numbers and look at the feasibility, and it looked promising. Samsung played an instrumental role in proposing and demonstrating mmWaves for mobile. That’s the 5G story from our side.

As for 6G, we found that the 5G technology kept getting better and better. We started looking at performance above 100 GHz a few years ago, even before creating the ComSenTer. We did a CMOS 240 GHz communication link to prove that the technology could work. You can communicate at tens of gigabits per second. We were constrained to CMOS at the time. We could see that CMOS limited us not so much in data rates but in output power above 100 GHz. We teamed up with Mark and formed the ComSenTer with the goal to push device, circuit, and system performance beyond 100 GHz with operating distances up to hundreds of meters.

The key is that we are now considering the entire problem, not just circuits or devices.  We are not designing circuits in isolation, but looking at the entire network. What will a mesh network of mm-wave look like? What’s the network architecture? That’s why we brought in NYU Wireless and Sundeep Rangan.

5GTW: When did UC Santa Barbara begin looking at 100 GHz?
Rodwell: UC Santa Barbara was unusual in that it committed to experimental work in semiconductors back in the 1980s when other universities were shifting to modeling. Since the 1990s, I’ve been working on high-frequency transistors, not in silicon but in other materials. We built our first 94 GHz amplifier in 1992 and had almost two hundred gigahertz in 1998 or so. It was akin to “let’s put a man on the moon because we can, not because we’d want to live there.”

There were a couple of DARPA programs in the late 1990s through the mid-00’s where we really pushed this. Working with John Zolper and Mark Rosker, who were at DARPA at the time, we wanted to push high frequency electronics. It started with “can you build a transistor that could run at more than 1 THz? Can you make the whole chipset and figure out packaging?” That became a DARPA program.  One team was at Northrop Grumman, led by Bill Deal, who is a member of our center’s scientific advisory board, while another team was Teledyne with UCSB and JPL. The Northrop people built a transistor which could work, theoretically, to 1.5THz, and then managed to build an amplifier that worked at 1.0 THz.  The Teledyne/UCSB/JPL team built a 1.1THz transistor and managed to build some pretty complex chips up to 650 GHz. So, we proved that such frequencies were possible, but given that the technologies were pretty exotic and expensive, what could you do with it?

With the evolution of CMOS, electronics became much less expensive. There were some limits on its power for longer range radio links. So, we at ComSenTer looked for useful things that we could do with the technology. At that point, we decided rather than continuing to push for higher speeds, let’s build something more complex and useful at these frequencies to drive real applications. “Use CMOS whenever you can” is a rule we tried to live by, but when you can’t use CMOS, use SiGe. Beyond that, use InP. While I’m really an InP person, GaN is becoming a mainstream technology. It can produce phenomenal amounts of output power.

5GTW: Why did ComSenTer researchers choose to study frequencies from 100 GHz to 300 GHz?
Rodwell: We chose to start at 100 GHz because there’s so much good work already done below 100 GHz. We’re being funded to look at technologies that are a few more years out. We’re limiting work to 300 GHz because atmospheric conditions make higher frequencies miserable to use. Unless you’re the Air Force or NASA, building links for high altitudes or for space, links above 300 GHz is fantasy. Even going above 150 GHz has been questioned. How many nines of reliability do you need? Under commercial service constraints, to get four nines reliability, from DC to about 300 GHz, the dominant worst-case weather condition is rain. But, that attenuation levels off when the signal wavelength equals the raindrop size. That’s at about 50 GHz. Once you reach 50 GHz, the atmospheric attenuation doesn’t get worse until about 250GHz. Higher than that, and the attenuation becomes very high when it’s humid. The question then becomes “can you build inexpensive electronics at 150 GHz?” You get large bandwidth and because the wavelengths are short, you can build an antenna array with lots of elements in a small space. An array can mathematically produce as many beams as it has antennas, though the signal processing people say divide that number by two. At 140 GHz, wavelength is 2 mm so element spacing is 1 mm. Thus, a 1 cm² array can have 100 elements. If you can get 100 beams, then you can use the allocated spectrum 100 times over.

5GTW: What are the drawbacks?
Rodwell: The penalty, however, is you get very short range, not only because of atmospheric attenuation but because of λ²/R². Our vision is to get complex electronics and use as much CMOS as we can. The output transistor is not likely CMOS because of output power. It’s likely SiGe, GaN, or InP.

Niknejad: When you go above 100 GHz, you have lots of bandwidth. You can easily build a system with 10 GHz of bandwidth and you can push a lot of data. We were able to push 80 Gbits/s over a single channel link. When you have that much data and you want to further increase capacity with a MIMO system, we think we can get to terabit/s data transfer. When you get to that high data rate using spatial multiplexing, how do you process that data? The problem of taking all that data, quantizing it, processing it, and shipping it, creating many problems along the way. For example, you’re limited by ADC power, DAC power, analog signal processing, digital signal processing, and high-speed links.

Rodwell: In addition to the high-speed circuits, we realize that the challenge comes in doing the back-end work of signal processing. The other problem is “how do we package all of this?”

Niknejad: The downside of using these high frequencies is that the signals don’t bend around corners. Most propagation is line of sight. You can have an outage problem. We are starting by measuring channels both indoor and outdoor, looking at short and long ranges. For example, we need to know how much delay spread is there, how much signal processing we need. How do we deal with the fact that the channel might be line of sight? The network handoff problem is important in 5G and will be more important in 6G. You can walk in front of a tree and lose the connection. The system won’t be robust unless you are connected to multiple base stations at one time. Tracking users and beam steering is much more difficult above 100 GHz. At 5 GHz, the beams are wide. You can track a user packet by packet. At these high frequencies, the signal is very weak without a beam. A base station is basically blind without the array gain and you don’t see the user locations. We need a clever scheme to quickly figure out which direction to send the information. You might have to do beam steering in both directions.

In 5G, a handset might have four antenna elements. A chipset can drive perhaps twelve antennas but only four at a time. The base stations are doing the heavy lifting of driving say 64 elements.

In our system, you can put more elements in a handset because of the shorter wavelengths. The problem of tracking gets more difficult because of the number of angles to search to find a user. It’s now gone up squared.

Rodwell: As far as back-end digital beamforming, it’s been a wonderful problem. Besides, Ali, and Elad Alon at Berkeley, we’ve got help solving these problems from Upamanyu Madhow at UCSB, Sundeep Rangan at NYU and Christoph Studer at Cornell. The first time we gave these talks about massive MIMO at 100 plus GHz, people stood up and said “you will need massive precision. Nonlinearity and phase noise will kill you.” A few months later, the team did some calculations. There are many signals coming in and phase errors will produce a lot of crosstalk. But, you also have many elements in your antenna array, which means you have lots of oscillators with their own random errors that sort of average the errors out. When you do the calculations, the phase noise problem doesn’t look so scary. We did the same calculations on the ADC resolution. We went back and forth on how many ADC bits we needed, but overall, it’s not as scary as we thought. We saw the same on linear dynamic range and third-order intermodulation distortion and gain compression.

As for MIMO, it’s very different at short wavelengths than at lower frequency. It’s much more beam like and so the number of paths between transmitter and receiver is smaller. In signal-processing terms, the channel is “more sparse.” Computationally, sorting out the channel turns out to be much less mathematically complicated. The ComSenTer has some efficient mathematical algorithms. If we map that to silicon, do we have something that can support 100 Gigabits per second for each of 100 cell phones? That’s 10 Tbps. The answer isn’t there yet, but it’s exciting. The complexity of these problems is looking less extreme than we once thought.

5GTW: How many digital beamforming channels can you support and at what data rate?
Rodwell: Think of it this way: If I have a hundred elements on a phased array antenna, I’ve got a matrix. Beamforming is inverting that matrix. If we were not using mmWaves and were using lasers, I’d simply look to see your location. The communication channel becomes a straight line. In that case, the problem of beamforming becomes a problem of imaging, which is computationally simple. As wavelengths get shorter, the problem becomes one of imaging, making the DSP less complicated.

One thing we have going in our favor is that there are plenty or test instruments on the market that can operate at 110 GHz to 170 GHz. The number is too large to explain in terms of sales to universities.

One thing that’s really encouraging: though we picked 140 GHz, 220 GHz, and 300 GHz as frequencies to research with a 10-15 year horizon, There are clearly quite a few industry people looking at 110 GHz to 170 GHz. So, the lower end of this frequency range isn’t as far away as people might think

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